In a double boiler over hot water, melt bittersweet chocolate chips and butter. In separate large bowl with electric mixer, beat eggs and sugar until thick and pale; stir in chocolate mixture. Stir together flour and baking powder and stir into chocolate mixture. Gently mix in semi-sweet chocolate chips and walnuts. On a sheet of plastic wrap, form dough into two equal logs, about 2 inches in diameter and 8 inches long. Wrap tightly and refrigerate at least 1 hour or until firm.